1. Field of the Invention
This invention relates to an exposure apparatus of the step-and-repeat or step-and-scan type and, more particularly, to a focussing method and system for controlling focussing and levelling between a substrate and an optical system of an exposure apparatus that is used to produce semiconductor elements and liquid crystal elements.
2. Background of the Related Art
In a step-and-repeat or step-and-scan exposure apparatus, images of circuit patterns, drawn on a master plate, are transferred to a photoresist layer of a semiconductor wafer and are developed to obtain a desired photoresist pattern.
In the general manufacture of semiconductor elements, multiple layers of circuit patterns are laminated. Therefore, it is necessary to accurately align an optical image of a circuit pattern, to which exposure is to be made, to a circuit pattern already formed on a wafer. In order to ensure accurate patterning, an exposure apparatus is essential for providing a fixed focus (xe2x80x9cfocussingxe2x80x9d) from a reduced projection lens of an optical system to a surface of a wafer, as well as providing xe2x80x9clevellingxe2x80x9d of the fixed focus area. Conventionally, an illuminant having a wide area is used to assure the levelling and, recently, a levelling control system has been used to assure two foci or more.
Semiconductor devices are becoming increasingly integrated. In both memory and non-memory fields, a stack structure has been used to increase the integration level. As the integration level increases, the stack structure becomes more complex. When a semiconductor integrated circuit having a complex stack structure is fabricated, an exposure apparatus must overcome different step heights in the complex stack structure so that the pattern has a consistent focus. If the exposure apparatus patterns respectively different foci, defective patterns are formed and production of the to semiconductor device becomes impossible. A conventional exposure apparatus for exposing a semiconductor device with this stack structure uses a focussing system in which the wafer surface is focussed by a capacitor or laser.
Referring to FIGS. 1A-1B, a conventional focussing system 200 assuring two foci or more has at least two light generating elements 210 and 220 and two light receiving elements 216 and 226. An incident angle (measured with respect to a vertical reference perpendicular to the plane of a wafer surface) of a first light 212 and a second light 222 created at the conventional light generating elements 210 and 220 is about 45xc2x0-60xc2x0, and transmittance of the first light 212 and the second light 222 is 80% or more. This conventional focussing system 200 detects a first reflective light 214 and a second reflective light 224 that are reflected at a first layer 256 and a second layer 254, respectively, which are stacked on a substrate 252 of a semiconductor wafer 250, to control focussing and levelling between the semiconductor wafer 250 and an optical system (not shown) for performing an exposure process. The first reflective light 214 and the second reflective light 224 penetrate a photoresist layer 258 of the semiconductor wafer 250 and then are reflected from the first layer 256 and the second layer 254, respectively. At this time, there is a step height difference between the first layer 256 and the second layer 254. Accordingly, while confirming a focus thereof, a light path difference xe2x80x9cAxe2x80x9d is created, as shown by the second reflective light 224 reflected at the second layer 254 in comparison with the first reflective light 214. Reference numeral 228 indicates the light path that would have resulted if there were no step height difference in the successive layers of the structure. In the conventional focussing system 200, as shown in FIG. 1B, the position of the semiconductor wafer 250 is controlled to overcome this light path difference between the first reflective light 214 and the second reflective light 224. However, this leads to incorrect levelling.
In other words, in a conventional focussing method and system, focussing and levelling are controlled by detecting a reflective light that penetrates a photoresist layer and then is reflected from a reflection layer such as a metallic layer and a poly layer formed under the photoresist layer. However, this method and system lead to incorrect levelling of a fixed focus area.
Moreover, achieving an adequate focus is made more difficult in the conventional focussing method and system because light (not shown) reflected from a surface of the photoresist layer 258 serves as noise which influences the reflective lights 214 and 224.
In the conventional focussing method and system, the step height difference created by a complex stack structure produces defective patterns. Particularly, in a focussing system assuring two foci or more by means of a laser, a stack structure to generates asymmetric or incorrect focus, preventing a consistent focus in a fixed area from being obtained. When the fixed area is exposed, a defocus is created and defective patterns are formed.
An object of the invention is to provide a focussing method and system which can obtain stable focussing and levelling of a substrate that has a complex stack structure and is exposed in an exposure apparatus for producing semiconductor elements and liquid crystal elements.
Another object of the invention is to provide a focussing method and system which can ensure a consistent focus in a fixed area of a semiconductor wafer having a stack structure in a focussing system confirming two foci by means of a laser.
According to a feature of the invention, there is provided a focussing method for controlling a distance between an optical system and a substrate in an exposure apparatus. The substrate has a surface coated with a coating layer so as to form a pattern on the substrate. The method comprises transmitting a light for detecting a focus on the substrate, wherein the light is transmitted at an incident angle at which an amount of light reflected from the surface of the coating layer is larger than an amount of light penetrating the surface of the coating layer; detecting the light reflected from the surface of the coating layer on the substrate; and controlling a position of the substrate in response to the detected light, thereby obtaining a consistent focus in a fixed area along the surface thereof.
In this embodiment, an incident angle of the light is equal to or greater than an angle at which reflectance of a component vertically entering the surface of a photoresist layer is higher than transmittance thereof.
In this embodiment, an incident angle of the light is equal to or greater than an angle at which reflectance of a component horizontally entering the surface of a photoresist layer is higher than transmittance thereof.
According to another feature of the invention, there is provided a focussing system for confirming two foci or more when an exposure process is performed on a semiconductor wafer having a stack structure. The system includes a light source for creating a light for detecting a focus of the semiconductor wafer, and then transmitting the light at an incident angle at which an amount of light reflected from the surface of the photoresist layer is larger than an amount of light penetrating the surface of the photoresist layer. A detection part detects the reflected light and then generates a detection signal. A control part controls a position of the semiconductor wafer in response to the detection signal generated at the detection part and confirms two foci or more, thereby providing a consistent focus in a fixed area of the semiconductor wafer.
In a focussing method and system of the invention, a light for measuring a focus is transmitted to a wafer with a fixed incident angle having a high reflectance and then light reflected from the surface of the photoresist layer on a semiconductor wafer is detected, thereby controlling a focus. Since a focus can be maintained in a complex stack structure on a semiconductor wafer, improper focussing and levelling is prevented during an exposure process, thereby assuring good patterning. In particular, it is possible to stably assure a consistent focus in a fixed area of a semiconductor wafer having a stack structure in a focussing system confirming two foci or more by means of a laser.